Figure: Information from detailed geological analysis populates a sequence stratigraphic framework so leading to accurate interpretations of depositional setting and predictions of lithofacies geometries in unknown portions of a basin.

Introduction

Lyell’s premise in 1830, 1832 & 1833 that “the present is the key to the past” is fundamental to sequence stratigraphy's recognition that the sedimentary record of the earth’s crust is the product of uniform and common physical processes that interacted with sediments as they accumulated. This means that the processes responsible for known portions of a geological section can be determined by careful description and analysis focused on component lithofacies, fabric and geometry. The origins of these sediments are then be interpreted by comparison with observations of similar features in modern sedimentary systems and their processes, results of flume experiments, computer simulations that re-create the fabrics and geometries seen in the field and laboratory, and the body of known geological information geologists have amassed in the geological literature. These interpretations are then integrated with the sequence stratigraphic framework of erosional and depositional surfaces that enclose and subdivide the section. This template extends the interpretation of the depositional setting and predictions of lithofacies’ geometries away from the known areas. It also aids prediction of sedimentary rocks likely to contain both hydrocarbon and water resources and what their characteristic fabrics might be.

This portion of the web site is based on the geologic literature, in particular Catuneanu, et al, (2011) and its summary of a collective understanding of sequence stratigraphy. This section defines and explains the origins of:

The relationship of the different depositional systems, with their different genetically related stratigraphic elements is then described in terms of the above sequence stratigraphic geometric end members. Further consideration is given to the stratal stacking patterns of different depositional systems combined to define trends in geometric character and systems tracts. The text explains how the framework of the elements of each depositional setting has common hierarchies that enable reconstruction of the sedimentary section and the prediction of lithofacies and paleogeography away from control points. The sources of information in this text are referenced, though if inadvertent omissions occur, it is likely the information was thought to be axiomatic.

Bounding surfaces and architectural elementsThe interpretation of the depositional setting of sedimentary strata is enhanced through understanding the origin of the character of the bounding surfaces to the sedimentary geometries of the component lithofacies. The bounding erosional and depositional surfaces of sedimentary geometries have hierarchical order. For instance partings with a high frequency include those of that subdivide shales, while bedding planes, the features commonly used to interpret the origin of the sedimentary section, are of lower frequency. In contrast lower frequency bounding surfaces include unconformities (Hutton, 1788), whose stratigraphic importance increases where they subdivide sedimentary rocks of varying ages.Brookfield (1977) was among the first known geologists to apply hierarchical order to surface boundaries subdividing sedimentary rocks, using his perspective of surfaces identified by Stokes (1968) in aeolian sediments. Brookfield (1977) identified:

First order boundaries that cut across underlying aeolian sediments when the migration of “draa” dunes occurred.

Allen (1983) describing fluviatile systems extended this by recognizing that bounding surfaces may be non- erosional or erosional. Using this he identified four surfaces:-

Concordant -non-erosional (normal bedding)

Discordant- non-erosional reactivation surface

Concordant - erosional

Discordant - erosional contacts

Studying braided streams, Allen (1983) used these surfaces to associate at least eight geometric shapes with specific lithologies and fabrics that he named "architectural elements”. Miall (1985) utilized this concept of depositional architectural elements to further classify and communicate information on the character and origins other fluvial depositional systems.

The application of the concepts of architectural elements is now widely used for most depositional systems. For example Pickering et al (1998) subdivided deeper water sedimentary bodies, recognizing a hierarchy of enveloping boundaries that genetically related discrete stratigraphic “architectural elements", "bodies", or "units" or “groups”. Others, including Sprague et al 2008, used a top-down hierarchical classification of "architectural elements" for deep-water settings that starts at a sedimentary basin scale. Sub-divided downward these form a series of broad elements includes the larger stacked channel complexes, in turn subdivided ever downward to an ultimate subdivision of laminae or even the individual sand grain. This top down classification is used to provide a framework of the basin to its interrelated broader larger scale "architectural elements" and their tie to the smaller scale "architectural elements". This can be inverted from small to large equally effectively.

A sequence stratigraphic analysis will iteratively use mixes and matches of a top down classification with a bottom-up classification. This interactive approach uses the general to guide an understanding of the specific and vice versa.

Below are further examples of the hierarchical architectures of different sedimentary systems. Click on the thumb nail and right click the shadow box image to see full size image!
This will work on most of the images on site.

The subdividing "surfaces" of the sequence stratigraphic framework that envelopes and encloses discrete geometric bodies of sediment establishes their order of accumulation from oldest to youngest. Interpretation is conducted by dis-assembling (backstripped) sedimentary bodies and then reassembling them in order in which they formed. The depositional setting is determined through the iterative reassembly and a consideration of the origin of the subdividing surfaces, geometry, lithofacies and fauna and evolving character. ‘‘Each stratal unit is defined and identified only by physical relationships of the strata, including lateral continuity and geometry of the surfaces bounding the units, vertical stacking patterns, and lateral geometry of the strata within the units"(Van Wagoner et al., 1990).

In addition the process of interpreting the origins of these surfaces, the depositional setting and gross sedimentary geometry of the rocks enclosed within the sequence stratigraphic framework involves Niels Steno’s Laws of Superposition and Walther's Law. The latter proposes that a vertical succession of sedimentary facies likely accumulated in adjacent depositional settings whether within a parasequence, system tract or a sequence. Paradoxically the surfaces used to subdivide stratigraphic sections are diachronous but in the process of interpretation this is oversimplified and the diachronous character of the surfaces is essentially ignored. For instance Holbrook and Bhattacharya (2012) indicate sub-aerial unconformities in fluvial systems meet these criteria but suggest these boundaries can still be used to bound systems despite intense diachroneity. The results of the use of the sequence stratigraphic methodology is that interpretation of depositional setting and a prediction of gross sedimentary geometry are confirmed in the field and with subsurface data.

Without exception all these surfaces, and in many cases the zones inferred to contain them and the sediment they enclose, transgress time, in other words are diachronous and in some cases may not even have the regional extent proposed for them, and so may be miss-correlated. Also the sequencestratigraphic surfaces have become largely conceptual surfaces imposed upon tangible rocks (Helland-Hansen, and Martinsen, 1996; Catuneanu, 2006; Embry et al., 2007; Miall, 2004; Holbrook and Bhattacharya, 2012). For instance Holbrook and Bhattacharya (2012) point out that the subaerial unconformity is more often than not is a conceptual surface and is assumed to be an approximate time barrier that includes the defining traits of originating as a ‘subaerial erosional surface’ preserved as an ‘unconformity that separates younger from older strata with significant hiatus.

As a result all the sequencestratigraphic surfaces, (SB, BSTSST, TS, and mfs) often violate Walther's Law, since they record shifts in facies deposition during transgression and regression and/or rates of change of accommodation, particularly at basin margins and along strike (Catuneanu, 2006). It is argued here that while most of these sequencesurfaces do not exactly fit their defining characteristics they can be mapped and bound enclosing facies that accumulated over a generally short time. The conceptual character of a surface is likely to be more so with increasing hierarchical rank (Catuneanu, et al, 2011). Despite these caveats sequencestratigraphic surfaces are useful for general "fuzzy" oversimple correlation. As interpretive tools they are commonly used and are often referred to in the stratigraphic literature. To conclude the definition of these surfaces is oversimplified and form discrete boundaries that can be traced beyond the scale of a single valley or comparable local depositional system, and used to make accurate facies predictions.Contention arises from the nomenclature of each of the sequencestratigraphic surfaces and the bodies they contain. This argument is based not so much on the constantly changing nomenclature as the developing understanding of sedimentary systems and their interpretation. However it is unfortunate that, though changes in nomenclature are well intentioned, these changes often add further to the confusion to a scientific methodology already weighed down with complex multi-word and multi-syllable terminology. Sequence Stratigraphic UnitsThe gross sediment geometric end members are represented by sequences, systems tracts, and parasequences are a hierarchy of stratigraphic packages or units of similar sediment strata whose geometries are of increasingly higher frequency, and are related to changes in the space or accommodation available for sediment fill; accommodation driven by changes in eustasy and tectonics (Jervey, 1988). It is shown how the geometric hierarchy is expressed in these packages by the subdividing and enveloping surfaces found in sedimentary sections. These bodies and their lithofacies are keys to determining and interpreting the depositional setting of the sedimentary sections that contain these bodies. It is contended that if depositional systems are described in terms of the geometric hierarchy of their lithofacies and elements this leads to a better understanding of the depositional origins of similar sedimentary bodies in the rock record.

Catuneanu (2009 and 2011) felt that the various types of sequence should be encompassed by the definition. They redefined a sequence as “a succession of strata deposited during a full cycle of change in accommodation or sediment supply”. The definition is generic, model-independent, and embraces the sequences listed above that may develop at any spatial or temporal scale. The requirement that a sequence coincide with a full stratigraphic cycle means that a sequence can be distinguished from component systems tracts. Existing sequence stratigraphic schemes incorporate a full cycle of change in accommodation or sediment supply with a beginning and the end of one cycle manifest by the same kind of event. This is the onset of a relative sea-level fall; the end of relative sea-level fall; the end of regression; or the end of transgression. In contrast, the boundaries of any systems tract correspond to different 'events’ within a relative sea-level cycle. The definition of a sequence is updated to be the fundamental statal unit of sequence stratigraphy (Catuneanu et al., 2011). As with Vail, et al., (1977) they see this as represented by a relatively conformable succession of genetically related strata bounded by surfaces but extend this to correspond to a full cycle of base-level changes or shoreline shifts depending on the sequence model being employed.

The Posamentier et al.'s 1988 original interpretation was that sediments accumulated during the falling stage of sea level cycle and this was where the sequence boundary should fall. Hunt & Tucker, (1992), 1995) discuss the role of forced-regressions and where the sequence boundary should be placed with respect to sea level position. Hunt believes that the position of the sequence boundary should be placed at the lowest position reached by sea level. A number of geologists support this contention. One of these is Pomar (1991) who recognizes that within the Late Miocene reefal platform of Mallorca, the sequence boundary and the downlap surface are both coeval and formed during the falling stage of sea level. Both surfaces bound the offlapping systems tract and merge landward in the erosion surface and, basinward, in the condensed interval. Note the correlative conformity on the top of the basin floor fan as suggested by Vail, 1987, versus the Hunt and Tucker, 1992 & 1995, models. Systems tractsA systems tract is a subdivision of a sequence independent of spatial and temporal scales representing “a linkage of contemporaneous depositional systems” (Brown and Fisher, 1977). It consists of a relatively conformable succession of genetically related strata bounded by conformable or unconformable sequencestratigraphic surfaces with an internal architecture that varies from a succession of facies that include high-frequency cycles driven by orbital forcing to a parasequence set or a set of higher frequency cycles. Systems tracts are interpreted on the basis of stratal stacking patterns, position within the sequence, and types of bounding surface (Van Wagoner et al., 1987, 1988, 1990; Posamentier et al. 1988; Van Wagoner 1995; Posamentier and Allen 1999). Systems tracts may be either shoreline-related, where their origin can be linked to particular types of shoreline trajectory, or shoreline-independent, where a genetic link to coeval shorelines cannot be determined (Catuneanu, 2011).

The FSST is formed by forced regressive deposits that accumulated after the onset of a relative sea-level fall and before the start of the next relative sea-level rise. The FSST lies directly on the sequence boundary sensu Posamentier and Allen (1999) and is capped by the overlying lowstand systems tract (LST) sediments. Hunt and Tucker (1992) differ with this placing the sequence boundary above the FSST, where this boundary marks the termination of one cycle of deposition and the start of another. Depending on the gradient of the depositional profile, the rate of sediment supply, and the rate of relative sea-level fall, a variety of 'attached' or 'detached' parasequencestacking patterns can be produced (Posamentier and Morris, 2000). Catuneanu (2011) explain that the terminology applied to this systems tract varies from 'forced regressive wedge' (Hunt and Tucker 1992) to 'falling sea-level' (Nummedal 1992) and 'falling-stage' (Ainsworth 1994). The simpler 'falling-stage' has been generally adopted by more recent work (e. g., Plint and Nummedal 2000; Catuneanu 2006). This systems tract has also been termed the early lowstand systems tract (Posamentier et al. 1988; Posamentier and Allen, 1999). The fall in relative sea level is evidenced by the erosion of the subaerially exposed sediment surface updip of the coastline at the end of forced regression, and the formation of a diachronous subaerial unconformity that caps the highstand systems tract (HST). The subaerial unconformity may be onlapped by fluvial deposits that belong to the lowstand or the transgressive systems tracts. The subaerial unconformity may also be reworked by a time-transgressive marine ravinement surface overlain by a sediment lag.

Regressive System Tract (RST)The RST lies above a TST and is overlain by the initial transgressive surface of the overlying TST. The complete sequence is known as a Transgressive-Regressive (T-R) sequence (Johnson and Murphy 1984; Embry and Johannessen 1992). The sediments of this systems tract include the HST, FSST and LST systems tracts defined above. There are cases where the data available are not sufficient to differentiate between HST, FSST and HST systems tracts. In such cases the usage of the regressive systems tract is justified. However, where permitted by data, the differentiation between the three types of regressive deposits (highstand, falling-stage, lowstand) is recommended because they refer to different stratal stacking patterns; are characterized by different sediment dispersal patterns within the basin; and consequently are associated with different petroleum plays. The last aspect relates to one of the most significant applications of sequence stratigraphy, which is to increase the resolution of stratigraphic frameworks that can optimize petroleum exploration and production development.Shoreline-Independent Systems TractsShoreline-independent systems tracts are stratigraphic units that form the subdivisions of sequences in areas where sedimentation processes are unrelated to shoreline shifts. These systems tracts are defined by specific stratal stacking patterns that can be recognized and correlated regionally, without reference to shoreline trajectories (Figs. 9–12). In upstream-controlled fluvial settings, fluvial accommodation may change independently of changes in accommodation at the nearest shoreline and create sequences and component low- and high-accommodationsystems tracts (e. g., Shanley and McCabe 1994; Boyd et al. 2000). Shoreline- independent systems tracts may also be mapped in deep-water settings controlled by sub-basin tectonism (e. g., Fiduk et al. 1999), but no nomenclature has been proposed for these situations. The timing of shore-line-independent sequences and systems tracts is commonly offset relative to that of shoreline-controlled sequence stratigraphic units and bounding surfaces (e. g., Blum and Tornqvist 2000).

This change in grain size is often caused by the abrupt changes in energy that are associated with the waves or currents of the sea transgressing across the sediment interface. These abrupt changes in grain size that bound a parasequence can be identified in well logs, outcrop and seismic and used to identify a parasequencecycle. Examples of these grain size changes can be seen in the parasequences of tidal flats, beaches, and deltas.

A parasequence in its original definition (Van Wagoner et al. 1988, 1990) is an upward-shallowing succession of facies bounded by marine flooding surfaces. A marine flooding surface is a lithological discontinuity across which there is an abrupt shift of facies that commonly indicates an abrupt increase in water depth. The concept was originally defined, and is commonly applied, within the context of siliciclastic coastal to shallow-water settings, where parasequences correspond to individual prograding sediment bodies.

In carbonate settings, a parasequence corresponds to a succession of facies commonly containing a lag deposit or thin deepening interval followed by a thicker shallowing-upward part, as for example in peritidal cycles. In contrast to sequences and systems tracts, which may potentially be mapped across an entire sedimentary basin from fluvial into the deep-water setting, parasequences are geographically restricted to the coastal to shallow-water areas where marine flooding surfaces may form (Posamentier and Allen 1999). In the case of carbonate settings, peritidal cycles can in some cases be correlated into slope and basinal facies (e. g., Tinker 1998, Chen and Tucker, 2003).

Figure: Hierarchy of cyclicity. Each stratigraphic element is a component of the subsequent lower-order element. Specific interpretations from McKittrick Canyon were used to construct the sequence stratigraphic framework from Tinker (1998).

For this reason, it has been proposed that a parasequence be expanded to include all regional meter-scale cycles, whether or not they are bounded by flooding surfaces (Spence and Tucker 2007; Tucker and Garland 2010). However, following the principle that a sequence stratigraphic unit is defined by specific bounding surfaces, many practitioners favor restricting the concept of parasequence to a unit bounded by marine flooding surfaces, in agreement with the original definition of Van Wagoner et al. (1988, and 1990).

We recommend the use of the sequence stratigraphic methodology to the analysis of any small, meter-scale cycles, as long as they display depositional trends that afford the recognition of systems tracts and diagnostic bounding surfaces. Parasequences consist of normal regressive, transgressive and forced regressive types of deposit, and display various stacking patterns.

Parasequences may be stacked in an upstepping succession, in which case they consist of normal regressive and transgressive deposits that accumulate during a period of positive accommodation in response to variations in the rates of accommodation and/or sediment supply. Upstepping parasequences may either be forestepping or backstepping (see the figure below).